reducing output
 

Reducing the output from 2" to 1-1/2" will cause the pump to work harder to pump the water up. The further away from the pump the reduction is the better.

Putting a reducer in is equivalent to increasing the head pressure and will greatly reduce the flow rate. From 2| to 1-1/2" pipe you will reduce the max possible flow (at the same pressure) by about 40%. (flow is proportional to pressure x area)

While friction losses are definitely increased with reduction in pipe size the amount is also dependent on flow rate. So to say one will experience a 40% reduction is not correct.

Chances are with you branching off some flow prior to going upstairs you won't even notice a difference between 1.5 & 2". 1.5" sounds like the right size to me.

Guess I should have clarified my response some.

In a hydraulic system if all other values are kept the same, including pressure and friction, the volume of flow will be reduced by the same ratio as the area. There are many other factors that can and will contribute to this, if you are looking to maintain maximum pump life and flow rate, keep the system restrictions to a minimum and as far away from the pump as possible, the longer the pipe is held at 2" the better.

In most pump driven system flow rate is maintained closer than the volume reduction due to increased pressure at the pump. Increasing the pressure increases the work the pump must do to maintain the flow rate and therefore reduces the maximum life expectancy.

If you are going to tee off do so before the reduction to reduce pressure at the pump and maintain pump life.